A differential gear to be used in a drive mechanism of a vehicle is one of differential devices used for a shaft connecting drive wheels of the vehicle to absorb a speed difference between an inner wheel and an outer wheel when the vehicle goes around a curve.
To be brief, the differential gear includes a ring gear attached to the outside of the differential case, a pinion gear placed in and attached to the inside of the differential case, and a gear engaged with the pinion gear and mounted on an axle.
Drive power generated by an engine and others in the vehicle is transmitted to the ring gear fastened to the differential case, thereby rotating the gear mounted on the axle through the pinion gear attached to the differential case, thus transmitting the power to the axle.
As another differential device to be used in a vehicle, there is a called LSD designed to compensate defects of a differential gear that causes idling of an axle while one of the wheels is in an unloaded condition. This LSD is similar to the above differential device in that a ring gear is provided on the outside of a differential case.
A method for fastening the ring gear to the differential case in the above differential device is conventionally executed by using bolts. However, this fastening method using bolts causes problems with an increase in weight due to the weight of bolts and the thickness enough to allow the fastening using bolts.
Therefore, a method for fastening by caulking is also studied instead of the fastening of the differential case and the ring gear with bolts (see Patent Document 1, for example). FIG. 8 is a cross sectional view of a conventional differential gear 101. FIG. 9 is a perspective view showing an enlarged part of a ring gear 103. FIG. 10 is a diagram showing a process for press-fitting the ring gear 103 onto a differential case 102. FIG. 11 is a diagram showing a process for fixing the ring gear 103 to the differential case 102 by caulking. FIG. 12 is a schematic enlarged view showing a caulking portion of the conventional differential gear 101.
The differential gear 101 shown in FIG. 8 will be mounted in a vehicle in such a manner that the ring gear 103 engages a drive gear (not shown) for transmitting drive power to the gear 103. The differential gear 101 is configured such that when the ring gear 103 fixed to one end of the differential case 102 receives a rotational torque from the unshown drive gear, the differential case 102 is rotated together with the ring gear 103. The differential case 102 has a mounting cavity 117 in which a differential gear 116 is housed. In the differential case 102, a pinion gear 119 is fixedly placed in the cavity 117 through a pinion shaft 118 and thus disabled from rotating. The pinion gear 119 meshes with the differential gear 116 to transmit drive power to an axle 120 connected to the differential gear 116.
The ring gear 103 is fastened to the differential case 102 by press-fitting the ring gear 103 onto the outer peripheral surface of one end portion of the differential case 102 and then caulking. As shown in FIG. 9, the outer peripheral surface of the ring gear 103 is formed with a gear section 104 which will receive a rotational torque. The inner peripheral surface of the ring gear 103 is formed with a plurality of notch portions 105 continuously arranged.
As shown in FIG. 10, the outer peripheral surface of the one end portion of the differential case 102 is provided with an annular press-fit surface 106 on which the ring gear 103 is press-fitted. The press-fit surface 106 is designed with an outer diameter larger than an inner diameter of the ring gear 103 to provide a press-fit allowance (interference). On a right end face of the press-fit surface 106 in the figure, a heel portion 107 is formed vertical to the press-fit surface 106 to thereby restrict a press-fit amount of the ring gear 103. On a left end face of the press-fit surface 106 in the figure, a flange portion 108 is formed extending from the press-fit surface 106.
The above differential gear 101 is assembled in a way that the ring gear 103 is fitted onto the press-fit surface 106 from the side of the flange portion 108 of the differential case 102 as shown in FIG. 10, and the ring gear 103 is press-fitted on the press-fit surface 106 until the ring gear 103 abuts on the heel portion 107. At that time, the ring gear 103 is press-fitted on the press-fit surface 106 so that the notch portions 105 are located on the opposite side from the heel portion 107. The flange portion 108 is then folded toward and pressed against the notch portions 105 as shown in FIG. 11. Thus, the material of the flange portion 108 is made to plastically flow in each notch portion 105. Accordingly, as shown in FIG. 12, the flange portion 108 is plastically deformed so as to enter in each notch portion 105, thus forming a caulking portion 110. The ring gear 103 is thus clamped between the caulking portion 110 and a contact portion of the heel portion 107 with the ring gear 103.